Today's architects want a feeling of "living space" in the high-rise apartments they design, but to achieve it they must have floor slab spans of 20 feet or more. In the past, architects have been forced to use thick reinforced concrete flat slabs, or slab on the ground framing. However, a new construction technique might be changing all that. Post-tensioning can offer architects a new dimension in design freedom since the longer spans greatly reduce the necessary number of columns. Post-tensioning also offers some advantages during construction. Post-tensioned slabs are 25 percent to 30 percent thinner and therefore require that much less material to be hoisted. A building which would ordinarily have 5 or 6 pounds of slab reinforcing bar per square foot can usually get away with about 1 pound of post-tensioning steel. And in a building of 20 plus stories, hoisting facilities are sometimes a critical factor. There are also savings in forming. Forms carry less load and thus can be lighter. Only enough re-shoring need be provided after stressing to carry the superimposed dead load from above. Most contractors use from one to four shores per bay. These savings can easily be seen in actual construction sites. On the West Coast, there have been a number of cases where a post-tensioned apartment building frame has been directly compared to a more traditional reinforced concrete design on the same project. In every case so far, the post-tensioning has proved more economical. Consider these factors: (1) each of these structures was a multi-million dollar project involving the best in architecture, engineering, and construction cost estimating. (2) In each case, the reinforced concrete design was prepared first, and thus the general building layout and structural scheme was such as to take full advantage of the inherent economies of that material. (3) Cost estimates on both alternatives were prepared by the same contractor. (4) The contractor's reinforced concrete estimate was the one which won him the job so it must have been figured very closely. (5) For architectural reasons, the redesign was not extensive enough to take advantage of a number of features that post-tensioning would have provided if used to begin with, such as smaller columns and shear walls, longer spans, and waterproof slabs. (6) The post-tensioned alternative involved the payment of an extra design fee, and still post-tensioning proved more economical.
Post-Tensioned Concrete for High-Rise Apartments
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